Known telecommunications networks operating using Wavelength Division Multiplexing include nodes to add or drop optical signals to or from the network. Such a node typically has two or more line directions for routing traffic. An optical cross connection within the node allows individual wavelengths carrying traffic to be routed on these different line directions. These known cross connections can also selectively terminate wavelengths.
A known reconfigurable optical add/drop multiplexer is shown in FIG. 1. A three-port optical add-drop node is shown generally designated 80. The device comprises a first port 12, a second port 14 and a third port 82 all in communication with one another. The three ports 12, 14, 82 share one multiplexer/demultiplexer module 42 having one bank of transponders 44. Thus, the ROADM can direct the components of the multiplexed signals between each of the ports and can drop selected components to the bank of transponders as well as add components. The first incoming signal 16 is split into three secondary signals 24, 34, 84 by the splitter 17. Each of these three signals 24, 34, 84 are substantially identical and continue on to a respective blocker 26, 46, 86 which can pass the signal 24, 34, 84 on command. A second incoming multiplexed signal 48 to the first port 12 is split into three secondary signals 50, 52, 87 by the splitter 54. The secondary signals 50, 52, 87 are substantially identical and continue on to respective blockers 56, 58, 88 which can selectively block the secondary signals on command. A third incoming signal 90 to the third port 82 is shown, which is split into three secondary signals 92, 94, 96 by a splitter 98. The secondary signals 92, 94, 96 are substantially identical. Each of the secondary signals 92, 94, 96 continues on to a respective blocker 10, 2, 4 which can pass the signal on command.
The three blockers 46, 56, 2 are then operated to selectively block two of the secondary signals 34, 52, 94. In this way only one of the signals 34, 52, 94 is passed to a shared splitter 60 which then passes the signal on to the shared multiplexer/demultiplexer 42 to be terminated at the shared transponders 44. The wavelengths of the secondary signal 24, 84, 50, 87, 92 or 96 that are not dropped by the transponders 44 are then allowed to pass through either of the blockers 26, 86, 58, 88, 10 or 4 respectively.
In a similar manner any new signals 62 added to the node 80 via the transponders 44 are first combined at the shared multiplexer/demultiplexer 42 and then passed to the shared splitter 60. The shared splitter 60 then splits the new signal 62 into three substantially identical secondary signals 64, 66, 6 which each continue on to a respective blocker 68, 70, 8. Depending on whether the new signal 62 is destined for the first port 12, or the second port 14, or the third port 82, the blockers 68, 70, 8 are then operated to selectively block two of the signals 64, 66, 6.
The architecture of this known node 80 permits the bank of transponders 44 to be shared by the three ports 12, 14, 82 for dropping and adding telecommunications traffic to and from the node 80. By activation of the blockers 26, 46, 56, 58, 68, 70, 86, 88, 10, 2, 4, 8 any one of the shared transponders 44 can selectively address either of the line directions. This is particularly advantageous due to the fact that, in practice, reconfigurable optical add/drop multiplexers are very costly due to the number of components from which they comprise.
Thus, the reconfigurable optical add/drop multiplexer of FIG. 1 utilises a combination of splitters and blockers to control the flow of traffic through the node. Alternatively, wavelength selective switches can be used instead of the splitter and blocker arrangement. This is advantageous as a wavelength selective switch has the means to selectively send signals to each of its outputs, as will be known to those skilled in the art. This reduces the number of components and optical fibre interconnections required.